(1) Field of the Invention
The present invention relates to the field of lubricating gearboxes, in particular power transmission gearboxes for a rotary wing aircraft.
The present invention relates to a dual circuit lubrication method and device with increased reliability for a mechanical system. The lubrication method and device are particularly suitable for lubricating a main power transmission gearbox of a rotary wing aircraft.
(2) Description of Related Art
A mechanical system generally has rotary elements, such as shafts and bearings, and also elements for transmitting power and for reducing or increasing speed, such as gearing. For proper operation of the mechanical system it is therefore essential for these elements to be lubricated and cooled, e.g. by means of oil under pressure. This lubrication is generally provided by a lubrication circuit and has as its main functions limiting the wear and the heating of the elements of the mechanical system, and consequently prolonging the lifetime of the mechanical system. Without such lubrication, the operation of the mechanical system may be degraded rapidly, or may become impossible.
As a result of the mechanical system being lubricated, the oil flowing through the lubrication circuit can sometimes become very hot, in which case it is cooled in a cooling circuit, which is generally located outside the mechanical system, prior to being usable once again for lubricating the mechanical system. The cooling circuit includes a heat exchanger, e.g. an oil/air heat exchanger.
The cooling circuit outside the mechanical system constitutes a vulnerable portion of a lubrication circuit for the mechanical system when it comes to leaks. The cooling circuit has pipes, numerous connections, and the heat exchanger. The cooling circuit is subjected to thermal stresses, such as a large difference between the temperature of the oil and the outside temperature, and it is subjected to vibratory stresses generated by the mechanical system and/or by a vehicle using the mechanical system. In addition, the cooling circuit is exposed outside the mechanical system. In particular when the mechanical system is fitted to an aircraft, the cooling circuit is situated outside the mechanical system of the aircraft, e.g. under a cover. Nevertheless, it can still be exposed to impacts against birds or ice, for example. As a result, one or more leaks may appear in its connections and its pipes and indeed in the heat exchanger, such leaks being caused essentially by the vibratory and thermal stresses.
When such leaks are present, the mechanical system can generally nevertheless continue to be lubricated, but only for a limited duration. It can happen that all of the oil stored in the lubrication circuit, e.g. in a tank, is lost to outside the lubrication circuit via such leaks. Such leaks may possibly be detected as a result of a drop in the pressure of the oil in the lubrication circuit.
Furthermore, a lubrication circuit also includes a pressure generator such as a pump in order to feed the lubrication circuit with oil and thus enable oil to flow in the lubrication circuit. In the event of a failure of the pressure generator, the flow of oil is interrupted, and consequently the lubrication of the mechanical system is likewise interrupted, with this occurring immediately.
In the event of this lubrication being lost, degradations can rapidly appear in the operation of the mechanical system. The consequence of such degradations occurring in a mechanical system fitted to a self-propelled vehicle may be that the vehicle ceases to be propelled immediately, or once the oil in the lubrication circuit has been used up.
In contrast, if the mechanical system constitutes the main power transmission gearbox of a rotary wing aircraft, such degradations of the circuit for lubricating the main power transmission gearbox can have consequences that are catastrophic, such as the aircraft performing an emergency landing or indeed crashing.
In order to mitigate such consequences, a mechanical system may include an emergency lubrication circuit. Such an emergency lubrication circuit makes it possible, in the event of the main lubrication circuit being out of operation, to provide lubrication at least for essential members of the mechanical system so as to ensure that the mechanical system continues to operate. For safety reasons, it is preferable for the aircraft to operate at a reduced level of power in order to limit the stresses on the mechanical system. Such an emergency lubrication circuit thus makes it possible, when the mechanical system is a main power transmission gearbox of an aircraft, for example, to enable the mechanical system to operate and consequently to enable the aircraft to operate in order to be able to reach a landing site. Such an emergency lubrication circuit thus improves the safety of the aircraft.
An emergency lubrication circuit may be arranged in parallel with a main lubrication circuit, as described in Document U.S. Pat. No. 8,230,835. Each lubrication circuit has its own pump, but they both use the same oil tank. Nevertheless, although the emergency circuit enables the mechanical system to be lubricated sufficiently in the event of a failure of the main circuit, it is in fact used only rarely. As a result, the emergency circuit constitutes an on-board mass that operates rarely.
In addition, the zones where leaks might appear in the main lubrication circuit are frequently situated in the heat exchanger and its connections. In order to avoid such leaks also appearing in the emergency lubrication circuit, the emergency circuit does not have a heat exchanger. As a result, the oil flowing through the emergency circuit is not cooled. Consequently, the emergency lubrication circuit can be used for a limited duration only in order to avoid the oil reaching a temperature that is too high.
The emergency circuit is generally put into operation automatically as a result of a loss of pressure being detected in the main circuit as a result of a failure of the pump of the main circuit or indeed as a result of a leak from the main circuit. The emergency lubrication circuit may also be put into operation manually by an operator.
In order to reduce the risk of a failure of the main lubrication circuit being caused by the emergency circuit, the emergency circuit is generally fitted with a bypass system serving to prevent flow in the pipes of the emergency circuit when oil pressure in the main lubrication circuit is sufficient. Thus, in the event of a leak from the emergency lubrication circuit, that circuit is no longer operational, but it does not prevent the main lubrication circuit from operating.
The drawback of the bypass system is that it makes it possible for there to be a dormant failure in the emergency circuit, with this failure being detected only at the moment when the emergency circuit is put into operation. Under such circumstances, there is a major anomaly that can be critical for a rotary wing aircraft.
In certain applications of rotary wing aircraft, the main power transmission gearbox does not include an emergency lubrication circuit proper, but rather it has two lubrication circuits that are identical and independent. Each lubrication circuit has its own pump and its own heat exchanger. Nevertheless, both lubrication circuits make common use of the same oil tank formed by the bottom of the main power transmission gearbox. As a result, if there is a leak in one of the lubrication circuits, the bottom of the main power transmission gearbox will be emptied of its oil over a shorter or longer period of time, thereby leading to a compete failure of the lubrication system.
Certain lubrication systems include an emergency tank, sometimes installed directly inside the mechanical system for lubricating, as described in Documents EP 2 505 878 and FR 2 658 577. The emergency tank is positioned above the members that it is essential to lubricate and it is fed continuously from the lubrication circuit. Oil then flows continuously under gravity from this emergency tank over the essential members. In the event of the lubrication circuit failing, the emergency tank is no longer fed with oil, but it enables the essential members to continue to be lubricated for a limited period corresponding to the time taken to empty the oil from the emergency tank.
Furthermore, Document U.S. Pat. No. 8,459,413 describes a different architecture that serves to limit the effects of a leak present in an oil tank. That architecture does not have an emergency circuit proper, but rather a pump that generates suction in the tank when a leak from the tank is detected. This suction then serves to limit the quantity of oil that is lost via the leak, thereby increasing the length of time the lubrication circuit can operate in spite of the presence of the leak. Nevertheless, the leak continues to be present and the length of time the lubrication circuit can operate is indeed increased, but it remains limited to until the oil has run out.
Also known is Document US 2013/0306022, which describes a lubrication system for an engine having two independent lubrication circuits. The flow rates and/or the pressures in the two circuits are different. The two circuits may lubricate different components of the engine or they may lubricate the same components so as to improve the effectiveness of the lubrication.
Furthermore, the technological background includes the following Documents: U.S. Pat. No. 4,976,335 and FR 2 826 094.